MCP6271-ESLASHST datasheet(14/34 Pages) MICROCHIP

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MCP6271/1R/2/3/4/5

DS21810E-page 14

4.6

Unused Amplifiers

An unused op amp in a quad package (MCP6274)

should be configured as shown in Figure 4-6. These

circuits prevent the output from toggling and causing

crosstalk. In Circuit A, R1 and R2 produce a voltage

within its output voltage range (VOH, VOL). The op amp

buffers this voltage, which can be used elsewhere in

the circuit. Circuit B uses the minimum number of

components and operates as a comparator.

FIGURE 4-6:

Unused Op Amps.

4.7

Supply Bypass

With this family of operational amplifiers, the power

supply pin (VDD for single supply) should have a local

bypass capacitor (i.e., 0.01 µF to 0.1 µF) within 2 mm

for good, high frequency performance. It also needs a

bulk capacitor (i.e., 1 µF or larger) within 100 mm to

provide large, slow currents. This bulk capacitor can be

shared with nearby analog parts.

4.8

PCB Surface Leakage

In applications where low input bias current is critical,

Printed Circuit Board (PCB) surface leakage effects

need to be considered. Surface leakage is caused by

humidity, dust or other contamination on the board.

Under low humidity conditions, a typical resistance

between nearby traces is 1012

Ω. A 5V difference would

cause 5 pA of current to flow. This is greater than the

MCP6271/1R/2/3/4/5 family’s bias current at 25°C

(1 pA, typ.).

The easiest way to reduce surface leakage is to use a

guard ring around sensitive pins (or traces). The guard

ring is biased at the same voltage as the sensitive pin.

An example of this type of layout is illustrated in

Figure 4-7.

FIGURE 4-7:

Example Guard Ring Layout

for Inverting Gain.

For

Inverting

Gain

and

Transimpedance

Amplifiers (convert current to voltage, such as

photo detectors):

a) Connect the guard ring to the non-inverting

input pin (VIN+). This biases the guard ring

to the same reference voltage as the op

amp (e.g., VDD/2 or ground).

b) Connect the inverting pin (VIN–) to the input

with a wire that does not touch the PCB

surface.

Non-inverting Gain and Unity Gain Buffer:

a) Connect the non-inverting pin (VIN+) to the

input with a wire that does not touch the

PCB surface.

b) Connect the guard ring to the inverting input

pin (VIN–). This biases the guard ring to the

common mode input voltage.

¼ MCP6274 (A)

VDD

¼ MCP6274 (B)

VDD

Guard Ring

VSS

VIN–VIN+

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